Electromechanical amplifier



June 18, 1940. H. H. SMITH ELECTROMECHANICAL AMPLIFIER Filed Feb. 9,1938 Patented June 18, 1940 UNITED STATES PATENT OFFICE Howard H. Smith,Chicago, Ill., assignor to Peru Radio Laboratories, Inc., Peru, Ind, acorporation of Illinois Application February 9, 1938, Serial No. 189,613

10 Claims.

This invention relates broadly to amplifiers, and more particularly tothe electromechanical variety in which currents to be amplified aretranslated into mechanical movement for the purpose of actuating someform of mechanical current varying device, such as a microphone,connected in circuit with an auxiliary source of power whereby themechanical movement may be translated back through the current varyingdevice into current variations of increased magnitude.

Stated more particularly, an important object of the invention is animproved electromechanical amplifier of the type which includes anelectromagnetic element mechanically connected to a microphone such thatprimary or input signals of sufficient strength to operate theelectromagnetic device will effect operation of the microphone which isarranged in circuit with a local source of power and is capable ofeffecting current variations of considerable magnitude compared with theslight mechanical movements resulting from the input signal variations.

Another object of the invention is the provision in electromechanicalamplifiers, of improvements in certain structural features of theelectromagnetic as well as the microphone elements.

Another object is the provision in electromechanical amplifiers of thetype characterized, of certain improvements in the construction of amoving primary coil element and means for maintaining a magnetic fieldin which said primary coil may react responsive to current variationstherein, the coil being mechanically connected with a microphone in asecondary circuit.

Another object is the provision of a microphone structure of thecompound variety having a diaphragm of light-weight alloy to reduce itsinertia constants, with microphone elements ar ranged on opposite sidesof the diaphragm and within the peripheral margins of the latter suchthat the diaphragm may be rigidly attached to the axial end portions ofthe moving primary coil.

Another object is the provision of improvements in means for maintaininga magnetic field, said means including a magnetic structure of highefficiency and arranged to provide a mounting for the amplifierelements.

Another object is the provision of an improved moving coil structureincluding a spider mounting for the moving coil which in turn issupported on the aforesaid mounting provided by the magnetic means andwhich, in addition to its simplicity, is highly efiicient for itsintended purpose.

Another object is the provision in an amplifier of the type described ofa moving coil system including a tubular coil mounting attached at oneend to a compound microphone diaphragm, and spaced bracket members formounting the compound microphone With one of the bracket membersextending through cut-away portions in the tubular member so that themoving coil and the diaphragm may be mounted concentrically to permitshifting movement of the tubular member 10 or coil in oppositedirections to operate both elements of the compound microphone.

Other objects and novel aspects of the invention reside in theparticular combination and construction of parts for achieving theseveral 15 objects heretofore recited and in providing an amplifier ofsimple construction capable of full Wave amplification and of increasedfrequency range.

The invention will be described with refer- 20 ence to the accompanyingdrawing, in which:

Fig. 1 is a top plan view of the amplifier;

Fig. 2 is a side elevation viewed in the direction of lines 22 of Fig.1;

Fig. 3 is an enlarged fragmentary vertical me- 25 dian section alonglines 3-3 of Fig. 1; while Fig. 4 is a circuit diagram illustrative ofone application of the amplifier.

The particular improvements in the art of electromechanical speakerscontributed by the pres- 30 ent invention provide an amplifier capableof greater fidelity and wider frequency response than amplifiers of thisclass heretofore developed, the improved performance of the presentamplifier being due to certain features of construction 35 in providinga more eificient magnetic circuit while at the same time simplifying themagnetic structure, a light-weight moving coil system and means formounting the same for a wider frequency response of increased constancy,and a microphone arrangement including a diaphragm constructed from avery light-weight alloy and attached to the axial end of the voice coilin the region of the periphery of the diaphragm.

A preferred embodiment of the invention, as 45 seen in Fig. 3, includesa support comprised of a permanent magnet IU of cylindrical form andmagnetized in the direction of its major axis. This permanent magnet ifdesired, however, may be in the form of a rectangular block of the shape50 of the top pole plate seen in Fig. 1, but provided with an annularbore, and magnetized in the same manner. In the present instance, themember [0 may be considered a cylinder.

Mounted at one axial end. of the cylindrical and Hi. It is important toobserve that the pole p ate has a cut-away or open portion I"! whichover the free end portion iii of the pole piece i2 and in spacedrelation to the latter so as to provide an air gap in which the magneticin the magnetic circuit for the permanent gnet l-S may be concentratedfor purposes hereinafter appearing.

The backing plate 4! and pole plate M in assembler relation with themagnetic member |ll comprise a form of magnetic structure constituting asupport or mounting for the remaining eiements of the amplifier.Attention is called to the rectangular form of the top or pole plate Mas it appears in Fig. l, for example. Where this lorm of construction ispreferred, the magnetic member it! may be in the form of a cylinder ormay be simply a ring-like member with rectangularly arrange exteriorwall surfaces corresponding to the shape of the plates H and M.

It may be remarked at this juncture that the means for providing amagnetic field in the re- .i of the air gap ll may also take the form.of a winding about the pole piece [2, for example, and connected to alocal battery for the purpose or setting up a held of variable intensitydependng upon a regulation of the current flow and the In such a case,of course, a perman nt magnet |El might be replaced by unmagnetizedelement.

The moving coil system for the electromagnetic part of the amplifierincludes a tubular member if? in the form of a cardboard or fiber tubeof he lightest possible weight consistent with the requisite rigidity towithstand the range of mechanical stresses placed upon it. This tubularmember is of a diameter slightly larger than that of the pole piece 58and such that the tube may fit freely in the air gap between the polepiece and the surrounding margins of the opening of the pole plate M.Between its ends the tube If; is rigidly attached as at 29 to a spiderthe latter being formed of suitably processed paper or other fibrousmaterial impr-ssed with a plurality of undulations 22 by means of whichthe tube i9 is permitted freedom of movement in the direction of itsaxis but is allowed only a negligible freedom in other directions. Theconnection 28 between the spider and the tube i9 is preferably formed byapplication of a light-weight cement resistant to atmospheric changesand of a type which will not become crystallized or brittle or becomeloosened by the constant vibration of the coil system. Such cements arewell-known in the art and therefore not spc led herein. The importantthing is that the connection or attachment of the tube Hi to the tier orother suspending means 2| be such will assure maximum. response andreaction movement of the coil in the line of its axis.

The outer end of the spider is similarly cemented to the offset portion24 of a mounting ring 25 arrangedconcentrically with the pole piece |8on the outside of the top or pole plate it (see Fig. l). The ring 25 isclamped in place by the bracket or bri ge construction for the inicroiihone hereinafter to be described.

Near its lower end l9 the tube IQ for the voice coil is provided with aprimary or reaction winding 26 extending inwardly to the region of theend of the pole piece l8 so as to lie substantially within the air gapi7, this winding 26 being adapted for connection by light-weightflexible conductors 21 and 28 (Fig. 1) in the primary or input circuitso that current variations in the latter: will react in the magneticfield across the air gap I? to cause the entire moving or voice coilstructure including the tube 9 to shift or vibrate in an ial direction,correspondingly.

Referring still to Fig. 3, it will be observed that at its upper end $9"the voice coil tube I9 is attached at 29 to the peripheral portions of adi' h vgm This diaphragm is desirably of t weight consistent with therequicchanical durability for a particular class ice. and is preferablymade of a lightoy such as Dura-lumin. lb hragzn 36 is of annular shapecorreclslely to the diameter of the tube l9 and is attached to thelatter preferably by means oi a cement such as heretofore characterized,but may also be crimped onto the latter, if desired.

A microphone structure cooperable with the moving coil system includes amain bracket or bridge member 3| having offset end portions 32 providedwith belt holes through which extend bolts 33 rigidly securing thebridge to the main suppo t or mounting structure, particularly the topor pole plate Is, the bridge extending across the middle of the polplate with its under side spaced appreciably above the upper or free endof the pole piece |2. A cup-shaped stationary electrode 31 is secured tothe under side of the bridge by means of a central stud 35 extendthroughan insulating sleeve 36 for engagement with a connection lug 31 and anut 38, the under side of the electrode 34 being kept out of contactwith the bridge by an insulating strr 3Q extending thereacross.

A paced mounting portion or bracket in the form of a second bridgemember 40 is secured the under side of the main bridge 3| by means ofbolts 4| extending through offset end portions 42 in the bridge andthrough insulating sleeves 43 seated in holes in the bridge 3| forengagement with a connection lug 44 and nuts 45. It

should be observed that the particular upper end E9" of the tube |9adjacent the diaphragm 30 .ed with cut-out portions 46 permitting dgemember 40 to pass therethrough.

smaller bracket 40 constitutes a spaced mou -ting portion for the mainbridge to which is secured second cup-shaped microphone electrode l?riveted at 48 to the smaller bridge or bracket portion opposite thefirst-mentioned electrode 3 and on opposite side of the diaphragm 3|) tothe same. tween the cup portion and corresponding side of the diaphragmare two felt rings 49, and arranged between the diaphragm in each of thecup portions within these felt rings is a current varying element orsubstance, for example, carbon granules 50.

It will. be apparent that axial movement of the tube IE! will transmitcorresponding movement to the diaphragm 30, and the microphone elementon one side of the diaphragm will be ac- Fitting closely betuated in onemanner while the microphone element on the opposite side willsimultaneously be actuated in an opposite manner. Stated in other terms,when the diaphragm 30 is driven in one direction by the tube, the carbongranules on one side thereof will be compressed while those on theopposite side will be decompressed or loosened. Thus, the arrangement ofmicrophone elements provides a compound. microphone which may beconnected in accordance with the circuit diagram of l, for example, toprovide full wave amplification.

Referring to Fig. l, a simple circuit arrangement utilizing thisamplifier includes an input transformer which may be connected in aprimary or signal circuit with its output or sec* ondary 52 connectedwith the moving coil winding 26. The movable actuating element ordiaphragm is connected by a conductor 53 to one side of a battery 54,the opposite side of this battery being connected by a conductor 55 tothe center tap of a primary 56 of an output transformer, the otherterminals of the primary winding being connected respectively byconductors 5'! and 58 (through lugs 31 and M on the unit) with thestationary or cup-shaped electrodes 34 and M, respectively. Thesecondary 59 of the output transformer is connected to any utilizationmeans such as a loud speaker or the like.

It will be apparent that in accomplishing the objects of the inventionand particularly in providing a moving coil system and means formounting the same so as to achieve a maximum efiiciency in translatingelectrical impulses into mechanical impulses, that the specificallydescribed means may be varied within the spirit of the invention as itis defined by the appended claims. Moreover, in specifically settingforth herein a particular means for connecting the moving coil systemwith a microphone for full wave amplification so as to translate themechanical impulses back into electrical impulses of increased amplitudewithout sacrificing fidelity or frequency response, variousmodifications and rearrangements are possible without departing from theinvention or the call of the claims. Likewise, while I have endeavoredto set forth what I consider to be the most suitable and efficientmagnetic element construction and bridge means for attaching the movingcoil and microphone system together so as to provide a compact andrugged unit, other arrangements may achieve the immediate objects of theinvention as set forth in the claims. It is, therefore, my intention toinclude within the call of these claims all equivalent arrangements withrespect both to form and location of the improved amplifying meansdisclosed.

What I claim as new and desire to protect by Letters Patent of theUnited States, is:

1. An electromechanical amplifier including means providing a mounting,a planar-like spider supported on said mounting, a signal coil includinga tube portion secured between its ends to said spider with its axistransverse to the plane of the spider, means forming part of saidmounting and providing a magnetic field for said signal coil, amicrophone arranged on said mounting opposite an axial end of the signalcoil and including an annular actuating diaphragm o1 minimized weightrigidly secured adjacent its peripheral portions to peripheral endportions of said signal coil tube, said signal coil being connected in aprimary circuit and said microphone being connected in a secondarycircuit such that energy variations in the signal coil. will efiectaxial movement of the latter to move said diaphragm and actuate themicrophone to effect corresponding variations in said secondary circuit.

2. An electromechanical amplifier including a mounting constructed ofparamagnetic material and including means cooperable therewith providinga magnetic field, an input coil having a tubular. mounting connectedbetween its axial ends to a yieldable spider member constructed topermit movement of said coil including said tube in the direction of itsaxis, said spider having support on said mounting, a microphoneincluding a pair of spaced electrodes aligned in spaced relation withthe axis of said tube, a lightweight diaphragm disposed between saidelectrodes and having rigid attachment to the axial end portions aroundthe periphery of an axial of said tube, together with compressibleconductive material confined between said electrodes and said diaphragmfor activation by the dia phragm when said tube is moved responsive tocurrent variations in said coil, said coil being arranged for connectionin a primary circuit, said diaphragm and electrodes of the microphonebeing arranged for connection in a secondary circuit.

An electromechanical amplifier including a substantially cylindricalpermanent magnet having a paramagnetic backing plate mounted on oneaxial end thereof, a pole piece arranged coaxially within saidcylindrical magnet and at:- tached at one end to said backing plate, theopposite end thereof being free, a paramagnetic pole plate on theopposite axial end of said cylindrical magnet and having open portionsspacedly surrounding said free end of the pole piece whereby to providea concentrated magnetic field in the region of said free end, saidcylindrical magnet, backing plate, and pole plate constituting amounting, a light-weight tube having an end portion. fitted freely oversaid free end of the pole piece for axial movement between the latterand said pole plate, said tube being rigidly attached wound on said tubein the region of said free end of the pole piece for connection to aninput circuit, said. microphone being connected with a source of powerin an output circuit.

4. An electromechanical amplifier including a magnetic structure ofsubstantially cylindrical shape and having an axially extending polepiece with a free axial end, a primary coil mounted for axial movementtoward and away from said free end. of the pole piece, said primary coilincluding a tubular mounting having an axial end portion extendingbeyond the axial extremity of the pole piece, a conductive diaphragmmember of annular shape secured around its periphery to peripheralportions of said particular end of the tubular mounting, and a compoundmicrophone having one electrode mounted on a bridge member secured tosaid magnetic structure and bridged over said particular axial end ofthe tubular member, said electrode including current varying meansmounted between the same and the corresponding side of said diaphragm,and a second electrode secured to a second bridge memher on the oppositeside of said diaphragm, said second bridge member being attached to theunder side of said first-mentioned bridge memher, said second electrodealso having current varying means arranged between the same and thecorresponding side of said diaphragm, means for connecting saiddiaphragm and said several current varying means in a secondary circuitincluding a source of power, said primary coil being connected in aprimary circuit, current variations in which eifect axial movement ofsaid tubular member to actuate said several current varying means bymovement of said diaphragm.

5. In an electromechanical amplifier, means providing a support, abridge member mounted on said support and a smaller bridge membermounted on the under side of said first-mentioned bridge member toprovide spaced parallel bridge portions, microphone elements mounted inopposite relation on each of said spaced parallel portions and havingtherebetween acommon actuating diaphragm of annular shape withperipheral portions extending beyond said microphone elements, alight-weight tube secured at one axial end to peripheral portions ofsaid diaphragm such that axial movements of the tube will actuate bothmicrophone elements, said tube having cut-away portions near said axialend through which portions of said second bridge member pass, yieldablemeans secured to said tube and mounted on said support and soconstructed as to permit substantial freedom. of movement of said tubein an axial direction and negligible movement in other directions, aprimary winding on said tube near the end thereof remote from saiddiaphragm for connection in a primary signal circuit, said diaphragm andmicrophone elements being connected in a secondary circuit including asource of power, together with magnetic means mounted on said supportand projecting axially within said tube in the region of said windingfor concentrating a magnetic field in the region of said winding.

6. In an electromechanical amplifier, a lightweight annular diaphragmhaving independent microphone elements arranged on opposite sidesthereof for simultaneous actuation thereby, said diaphragm andmicrophone elements being arranged for connection in a secondary circuitincluding a source of power, a magnetic structure providing a support, abracket structure mounted on said support and including spaced portionsto each of which one of said microphone elements is attached with saiddiaphragm therebetween, a tubular member having cut-away portions near aparticular axial end thereof and fitted over a part of the spaced meansof said bracket structure with said particular axial end thereof rigidlyattached to said diaphragm such that said tubular member may be moved inan axial direction to actuate said diaphragm and said microphoneelements, spider means attached to said tubular element between itsaxial ends and secured to said support so as to permit freedom ofmovement of the tubular member in an axial direction for the purposeaforesaid, a primary winding on said tubular member adjacent an endthereof opposite said diaphragm, and magnetic pole means forming part ofsaid support and extended within said tubular member for the purpose ofconcentrating magnetic lines of force in the region of said winding,said amplifier including means arranged as a part thereof for producinga magnetomotive force inducing lines of force through said winding intosaid magnetic pole means.

'7. In an electromagnetic amplifier, a primary coil structure comprisinga coil and supporting tube therefor, said tube being attached at one ofits axial peripheral extremities to a diaphragm arranged as a commonoperating element between opposite differential electrodes of adifferential type microphone, said coil being movable principally inaxial displacement to actuate said diaphragm between said electrodes.

8. In a device of the class described, a microphone and primary coilmechanism which includes a microphone having asubstantially planaroperating element, a primary coil structure of tubular form attached atone axial periphery to said planar operating element with the axis ofthe coil concentrically normal to said planar element, and meansextended in a plane substantially parallel to that of said planarelement and supportably engaging said coil structure between its endsfor axial displacement.

9. In an. amplifier of the class described, a microphone and signal coilmechanism including a differential microphone having a pair ofdifferential electrodes with a common substantially planar actuatingelement arranged therebetween for coaction with the same and currentvarying means by movement of the planar member in opposite directionsnormal to its plane, and a signal coil structure of tubular shapeattached in the region of the periphery of one of its axial ends to saidplanar actuating element. and means supportably engaged with said coilstructure and arranged to permit axial displacement thereof.

10. In a moving coil device, a coil structure comprising a tubular coilcarrier and a coil thereon, said carrier having a diaphragm mountedrigidly in the plane of one of its axial ends, said diaphragm beingadapted for cooperation with an instrumentality to be operated bymovement of said coil structure, and means extending substantially in aplane transverse to the axis of said coil structure and supportablyengaged with the latter in a region between the ends thereof andconstructed and arranged to permit axial displacement of the coilstructure.

HOWARD H. SMITH.

